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Full text of "Methods of physical and chemical soil analysis"

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UNIVERSITY OF CALIFORNIA Agricultural Experiment Station 

College of Agriculture e. w. hilgard, director 

BERKELEY, CALIFORNIA 



CIRCULAR No. 6. 

(Junk, 1903.) 



Methods of Physical and Chemical Soil Analysis. 

By E. W. HILGARD. 

Revised from Bulletin No. 38, Bureau of Chemistry, U. S. Department of Agriculture, 1893, 
and reprinted for the use of the students in the College of Agriculture. 



Before entering upon the details of physical and chemical soil investigation, it may 
not be superfluous to define what are the objects to be accomplished in such investiga- 
tions, both from the standpoint of the farmer and of the agricultural expert. 

It is well understood that in the present state of knowledge, the chemical analysis of 
soils long cultivated and fertilized affords, alone, but a limited amount of information 
regarding their productiveness, although always useful in defining their general charac- 
ter and obvious deficiencies. In the case of those cultivated without fertilization, even 
for a considerable length of time, chemical analysis will afford us valuable indications 
not only regarding their general character, but also in respect to the important soil 
ingredients most likely to have been reduced below the level of profitable culture. Yet, 
so long as virgin spots fairly representing cultivated lands exist, the results obtained 
by the examination of the former can be fruitfully applied to the latter. 

In virgin soils the indications, as experience has shown, become so definite as to 
permit a close forecasting not only of the general character and value of lands not yet 
brought under cultivation, but also of their best adaptations and of the probable dura- 
tion of productiveness without fertilization. Chemical analysis can thus, when com- 
bined with the physical, geological, and botanical examination of soils, give information 
of such direct practical importance, that in the newer States and in the Territories it 
becomes a reliable guide to the settler in the choice of lands and cultures. Such work 
is therefore peculiarly the province of the experiment stations west of the Mississippi 
River. 

As regards physical soil analysis as practiced in Europe, its results have fallen far 
short of the expectations which, theoretically, it might be expected to fulfill, viz., the 
definite measurement of the "tilling qualities" and of other physical coefficients of 
soils. The judgment regarding these points deducible from the great majority of 
mechanical soil analyses as usually made and published, affords hardly more informa- 
tion than could be derived from mere hand tests on the dry and wet soil. Yet it is 
hardly doubtful that a proper study of the physical composition can be made to yield a 
much deeper insight into the functions of the several physical soil ingredients and, 
consequently, into the means of modifying the physical properties to the best advantage. 

One important function of such examinations (about the feasibility of which there 
can be no reasonable question) is the identification of uncultivated soils with those of 
lands already under cultivation, thus permitting the application to new lands, of 
experience already acquired. But in order to attain such results, not only must the 
mode of sampling in the field, the preparation of the samples for analysis, and the 
physical and chemical analysis itself, be conducted upon a well-considered and uniform 
system, but the conditions of occurrence, the "lay," depth, climate, natural vegetation, 



— 2 — 

etc., must be known as fully as possible, since all tbese factors must be taken into con. 
sideration in interpreting, for practical purposes, the results of the physical and chemical 
examination. Moreover, in determining the conditions to be fulfilled and the modus 
operandi, all arbitrary conventions should be avoided as much as possible, since all 
such are sure to be violated, sooner or later, by 'workers who consider themselves as 
much entitled to exercise of judgment as any one else. In developing the methods 
originally suggested by D. D. Owen, it has therefore been sought to establish a rational 
basis inherent in the nature of the case, as far as possible, and to determine maxima or 
minima rather than arbitrarily assumed means. The detailed motivation of all these 
points would increase this report far beyond the admissible limits; but references are 
given to the publications in which such points are discussed. 

For brevity's sake a detailed description of the manipulations in the case of well- 
known analytical methods is omitted, mentioning only some critical points upon 
which success depends, but taking for granted the following out of the accepted precepts. 

THE SAMPLING OF SOILS. 

Since the practical utility of soil work would be greatly impaired were it dependent 
only upon the personal exploration of the wide domain by the experiment station 
officers, I have formulated, and long used successfully, the following "Directions for 
taking Soil Samples," that are forwarded either to persons desiring information as 
to their lands, or to intelligent farmers residing in regions of which the soils are to be 
investigated. It is often surprising how accurate and graphic are the data so obtained. 

In taking soil specimens for examination the following directions should be carefully 
observed, always bearing in mind that the analysis of a soil is a long and tedious 
operation, which can not be indefinitely repeated: 

(1) Do not take samples indiscriminately from any locality you may chance to be 
interested in, but consider what are the two or three chief varieties of soil which, with 
their intermixtures, make up the cultivable area of your region, and carefully sample 
these first of all; then sample your particular soil with reference to these typical ones. 

(2) As a rule, and whenever possible, take specimens from spots that have not been 
cultivated, nor are otherwise likely to have been changed from their original condition 
of "virgin soils" — e. g., not from ground frequently trodden over, such as roadsides, 
cattle paths, or small pastures, squirrel holes, stumps, or even at the foot of trees, or 
spots that have been washed by rains or streams, so as to have experienced a noticeable 
change, and not to be a fair representative of their kind. 

(3) Observe and record carefully the normal vegetation, trees, herbs, grass, etc., of the 
average land ; avoid spots showing unusual growth, whether in kind or quality, as such 
are likely to have received some animal manure or some other outside addition. 

(4) Always take specimens from more than one spot judged to be a fair representative 
of the soil intended to be examined, as an additional guarantee of a fair average. 

(5) After selecting a proper spot, pull up the plants growing on it and brush off the 
surface lightly to remove half-decayed vegetable matter not forming part of the soil as 
yet. Dig a vertical hole, like a posthole, at least 20 inches deep; in arid regions, not 
less than 36 inches. Scrape the sides clean so as to see at what depth the change of tint 
occurs which usually marks the downward limit of the surface soil, and record it. Take 
at least half a bushel of the earth above this limit, and on a cloth (jute bagging should 
not be used for this purpose, as its fibers, etc., become intermixed with the soil) or 
paper, break it up and mix thoroughly, and put up at least a pint of it in a sack or 
package for examination. This specimen will, ordinarily, constitute the "soil." 
Should the change of color occur at a less depth than 6 inches the fact should be noted, 
but the specimen taken to that depth nevertheless, since it is the least to which rational 
culture can be supposed to reach. 

In case the difference in the character of a shallow surface-soil and its subsoil should 
be unusually great— as may be the case in tule or other alluvial lands or in rocky dis- 
tricts — a separate example of that surface soil should be taken, besides the one to the 
depth of 6 inches. 

(6) Whatever lies beneath the line of change, will constitute the "subsoil." But 
should the change of color occur at a greater depth than 12 inches the "soil " specimen 



— 3 — 

should nevertheless be taken to the depth of 12 inches only, which is the limit of ordi- 
nary tillage; then another specimen from that depth down to the line of change, and 
then the subsoil specimens beneath that line. 

The depth down to which the last should be taken will depend on circumstances. It 
is always necessary to know what constitutes the foundation of a soil, down to the 
depth of 3 feet at least, since the question of drainage, resistance to drought, etc., will 
depend essentially upon the nature of the substratum. But in ordinary cases in the 
humid region, 10 or 12 inches of subsoil will be sufficient for the purposes of examination 
in the laboratory. The specimen should be taken in other respects precisely like that 
of the surface soil, while that of the material underlying this "subsoil" may be taken 
with less exactness, perhaps at some ditch or other easily accessible point, and should 
not be broken up like other specimens, so as to preserve, e. g., the character of "hard- 
pan " 

In the arid regions, where surface soil and subsoil are very much less definitely differ- 
entiated, and the depth of the soil mass is usually very great, it is in most cases 
advisable to take samples representing the average of each foot, from the surface 
down to 4 or 5 feet ; or even more, according to circumstances. 

(7) Specimens of salty or " alkali " soils should, when practicable, be taken toward 
the end of the dry season, when they will contain the maximum amount of the injuri- 
ous ingredients with which it may be necessary to deal. 

Since ordinarily the alkali salts are contained mainly within the first 4 feet below 
the surface within which the salts descend or rise according to the seasons, it will for 
practical purposes mostly be sufficient to take an average sample of a 4-foot column, 
the leaching of which will indicate the kind and amount of salts to be dealt with. But 
the last portions brought up by the soil auger should at least be cursorily examined for 
their salt content, and therefore sent separately. In some cases it will be desirable to 
sample each foot of the column separately. 

(8) All peculiarities of the soil and subsoil, their behavior in wet and dry seasons, 
their location, position— every circumstance, in fact, that can throw any light on their 
agricultural qualities or peculiarities— should be carefully noted and the notes sent 
with the specimens. Unless accompanied by such notes, specimens can not ordinarily 
be considered as justifying the amount of labor involved in their examination. 

It will be noted that in these " directions " the depth of the surface " soil " sample to 
be taken is left to the judgment of the farmer, between the limits of 6 and 12 inches, the 
first being the minimum, the latter the maximum, depth to which rational culture 
usually reaches. 

When the soil sample is to be transmitted to any considerable distance it is very 
desirable that it should be dried in the sun as completely as possible before shipment, 
since otherwise not only does it often arrive in an abnormally puddled and compacted 
condition, but may become moldy, changing the natural condition not only of the 
organic but also of the reducible inorganic ingredients; notably that of ferric hydrate 
and nitrates. With respect to the latter the date of taking the sample is also desirable 
and should be recorded in every case. 

THE MECHANICAL OR PHYSICAL ANALYSIS OF SOILS. 

The "insoluble residue" of the chemical analysis indicates in a very general manner 
the amount of "sand " in a soil ; and when combined with a determination of the soluble 
silica, alumina, and lime we also gain some idea of possible plasticity or "heaviness." 
But these indications given by chemical analysis are far too indefinite for practical 
requirements It is to the mechanical (or physical) analysis that we must look for 
practically available data on the tilling qualities of soils; but the loose practice mostly 
prevailing thus far has rendered the results far from satisfactory. 

PRELIMINARY EXAMINATION. 

The first step toward the determination of the character of a soil sample should 
always be what might be termed the hand tests, to wit, the observation of the greater or 
jess degree of ease with which the soil lumps crush between the fingers in the dry con- 
dition ; the presence or absence of coarse sand, etc.; also the change of color on wetting, 



_ 4 — 

the degree of rapidity with which the water is absorbed, the extent to which it softens 
the lumps, and the degree of plasticity assumed when the wet soil is kneaded. 

The next step should be the at least approximate identification of the minerals forming 
the coarse portion, since it must be presumed that, as a rule, these represent the orig- 
inal nature of the fine grains also. We thus already gain a very close insight into the 
origin and general character of the soil. This mineralogical examination is advantage- 
ously combined with an approximate quantitative determination of the coarse portion by 
sedimentation in a small beaker, or more advantageously, by means of a "Kiihn's cyl- 
inder" and an upward current of water of definite velocity ; that of 2 mm. per second is 
convenient for this purpose, provided that the sample is kept stirred by means of a rod. 
The dried coarse portion is weighed and then examined with lens and microscope, with 
or without previous separation by means of Thoulet's or some other dense fluid. This 
examination is of especial value in the identification of the sample with other soils 
previously investigated, and thus frequently saves a very large amount of analytical 
labor. 

When the preliminary examination does not suffice for the purposes in view, and 
detailed mechanical or "silt analysis" must be resorted to, more elaborate and complex 
methods and appliances are called for. 

SEDIMENTATION AND HYDRAULIC ELUTRIATION. 

In a general way a soil may be considered as consisting of clay intermixed with more 
or less mineral powder or sand of various grades of fineness, together with some humus 
or vegetable mold. The two former usually determine, in the main, the tilling quali- 
ties of the soil. 

As regards, first, the conventional "clay" which figures in the analytical statements, 
e. g., of the German investigators, it oftentimes consists far more of fine silex than even 
of total kaolinite substance. Since the latter itself consists at least of two portions 
totally distinct in their physical functions, viz., of chalky crystalloid grains and of a 
" colloidal," highly diffusible, and extremely adhesive and hygroscopic portion, the pres- 
ence of which determines, in the main, the corresponding physical properties of the 
soil, the indefiniteness of the current mode of statement and consequent classification 
of soils is so great as to render a clearer and more rational and definite method of 
mechanical analysis absolutely necessary from this point of view alone. An additional 
source of error arises from the viscosity and consequently greater hydraulic efficacy of 
'' clay water," which carries off, at the same velocity, much larger particles of rock pow- 
der than does pure water ; or in the case of subsidence, permits them to settle much 
more slowly. The consequence is that the percentage of a sediment of a given 
" hydraulic value " will be found very different, according to the amount of colloidal 
clay present, or according as the latter has or has not been wholly or partially removed 
prior to the separation of the simply pulverulent or sandy ingredients. Adding to these 
sources of uncertainty the wide range of size of grain included and weighed as one sedi- 
ment by most observers, the slight value attaching thus far to the results of mechanical 
soil analysis as representing the farmer's experience is amply explained. 

For the separation of fine powders and mixed grain-sizes, sedimentation by stirring 
up in water and drawing off the suspended sediment with the water after a definite 
time allowed for the subsidence of the coarser portions is, of course, the simplest process, 
which has been used in the arts and in pharmacy for centuries. For analytical purposes 
it has the disadvantage that in order to effect a reasonably complete exhaustion of a 
sediment of a definite size or "hydraulic value," the operation of stirring up and drawing 
off must be repeated a large number of times, since the time of subsidence is reckoned 
for the top layer of water, while subsidence actually occurs from below as well. The 
subsidence method requires close and continuous attention in its execution for a length 
of time proportioned to the minuteness of subdivisions desired ; moreover, in the case 
of fine sediments, which have a tendency to coalesce (flocculate) under the action of the 
irregular currents caused by stirring, the difficulty of obtaining them properly segre- 
gated is almost insuperable, greatly detracting from the accuracy of the determinations. 
The subsidence or (as it has lately been termed) "beaker" method, however convenient 
on account of its simplicity, is therefore ill adapted to the carrying out of extended 
investigations requiring the performance of a large number of mechanical soil analyses, 



— 5 — 

and its laboriousness has long caused it to be replaced, in European practice, by various 
devices embodying, in a more or less perfect form, the idea of separation of grain-sizes 
by an ascending current of water, or "hydraulic elutriation." The writer's personal 
experience has brought him into full sympathy with this view. 

When, in 1872, the writer began the consideration of this subject in connection with 
the agricultural survey of the State of Mississippi, even a very superficial test led him 
to reject quickly the grossly inaccurate and misleading apparatus of Nobel, with its 
four vessels of ever-varying capacity and form and inconstant head of water. The best 
then known, that of Scheme, proved much more satisfactory, provided the "colloidal 
clay" was first removed. But even then there remained a large and variable residual 
error, shown in the mixed grain-sizes of the sediments and correspondingly varying 
percentage results. A protracted investigation of the causes of these inaccuracies, made 
in the years 1872 and 1873, and published in the latter year,* proved that the great diffi- 
culty encountered in the separation of soil ingredients by the ordinary methods of 
sedimentation heretofore employed has been in the strong tendency of the fine particles 
to coalesce into large, compound floccules, and settle with the coarser sediments. When 
violently shaken they part company and become diffused singly through the liquid, 
which then presents simply a general turbidity; the particles then settling down slowly 
and singly at the rate corresponding to their individual size or hydraulic value. 

The following experiment, well suited to the lecture table, serves to demonstrate the 
above principle: If a given quantity of pure siliceous sediment of, e.g., 1 mm. hydraulic 
value (which has therefore been carried off at the velocity of 1 mm. by an ascending 
current of water) be again placed in the same current, in a long conical tube with cylin- 
drical outlet above, a considerable portion will fail to pass off, and will gather into 
flocculent aggregates, revolving in the lower (narrow) part of the tube. If instead of the 
full velocity of 1 mm. only 0.8 mm. is used, none of the sediment will pass off, but after 
some time it will be found wholly gathered together into the heavy flocculent aggregates, 
when the full velocity of 1 mm. may be used without causing any considerable portion 
of sediment to be carried off, until by violent stirring with a rod the floccules are 
destroyed. It is only, however, by repeating this outside stirring a number of times, 
that it is possible to get nearly all the sediment corresponding to the velocity of current 
actually used to pass off, The stirring by the current itself is powerless to do so, because 
the return currents down the sides of a conical tube perpetually cause recoalescence or 
" flocculation." 

It is thus clear that even with purely siliceous and nongiutinous sediments correct 
results can not be obtained so long as conical elutriator tubes are employed; and this 
effectually bars the claims of, e. g., Schone's apparatus (now adopted by the German 
experiment stations), to the accuracy desirable in this work. 

But if cylindrical elutriator tubes are alone admissible, then it follows that agita- 
tion by outside power, for keeping the soil or powder in suspension and continuously 
resolving the floccules inevitably formed under the circumstances, is indispensable. 
However, the tendency to coalescence diminishes of course as the size of the grains 
increases, but does not altogether cease until their diameter exceeds 0.2 mm., or about 
16 mm . hydraulic value. For the elutriation of coarser sediments hydraulic stirring may 
be successfully employed. 

We may therefore formulate as follows the conditions to be fulfilled by mechanical 
soil analysis upon which definite conclusions as to the physical and agricultural (or 
"working") qualities of soils, and of the functions of the several grain-sizes in deter- 
mining the same, may be based : 

(1) The preliminary preparation of the soil sample must leave its natural physical 
condition unimpaired. It must, therefore, not be heated to any temperature likely to 
wholly or partially dehydrate any of its constituent colloids ; nor must it be triturated 
or "pestled" in any manner likely to destroy the naturally existing aggregates or 
floccules cemented by lime carbonate, ferruginous, zeolithic, or other cements; nor 
must the latter be dissolved by the use of acids, thus changing the natural aggregates 
into a multitude of fine particles which do not exist in the original soil, and perform 



*Amer. Jour, of Sci., Oct., 1873; Proc. Amer. Assoc. Adv. Sci., Portland meeting, 1873. See also 
The Flocculation of Particles," Am. J. Sci., March, 1879. 



— 6 — 

the physical functions of sand grains of corresponding size. Boiling the soil is free 
from these objections. 

(2) Prior to any attempt to separate the different grain-sizes, whether by the hydraulic 
or subsidence method, the "colloidal clay" must be completely removed ; and in view 
of the prime importance of the latter as a physical soil ingredient it must be deter- 
mined by direct weighing, and not merely by loss, or " difference " 

(3) If the hydraulic method be used for the separation of the successive grain-sizes 
this must be done in vertical, cylindrical elutriator tubes, provided with a device for 
mechanical stirring by outside power, for preventing and undoing the flocculation of 
the sediments into heavy aggregates of indefinite composition and hydraulic value. 

(4) The importance of discriminating between the several fine sediments in respect 
to their physical functions is so great that a much larger number of subdivisions of 
these must be made than is now usually done. This proviso renders the ordinary 
process of sedimentation by stirring and subsidence in cylindrical vessels (beaker 
method) practically inapplicable to any extended investigations requiring frequent and 
numerous mechanical analyses, since each one would take an amount of time and 
practice quite out of reach of ordinary laboratory personnel. The work must in the 
main be done automatically to be generally available. 

The ways and means by which these several conditions may most easily be fulfilled 
will now be considered in detail. 

PRELIMINARY PREPARATION. 

In some cases simple sifting will serve, without further preparation, to separate the 
original, dry soil sample into appropriate subdivisions, down to the fine earth that is to 
serve for detailed mechanical and chemical analysis. In most cases, however, a certain 
amount of mechanical disintegration must be resorted to in order to detach the earth 
from the larger sand grains and aggregates, and here some judgment must be exercised 
by the operator. A preliminary washing, aided by the lens and microscope if neces- 
sary, will show whether there are any soft concretions or decomposed rock particles 
likely to be crushed by a rubber pestle, the hardest material admissible, but which will 
serve admirably when no soft grains will be crushed by it, thus changing the nature of 
the soil to a corresponding extent. The liability to such change is much increased by 
wetting the soil, when calcareous and ferruginous concretions (bog ore) may be crushed 
to such extent as to destroy the value of the work. In the case of heavy clay ("adobe ") 
soils, however, wetting, and even hot digestion with water, may be necessary to even 
the preliminary disintegration serving to prepare a fair average sample. The slushy 
mass must then be economically washed through the fine-earth sieve and the remnant 
afterwards separated into sizes by sifting, while the fine-earth slush is evaporated and 
dried to serve for analysis. 

Since a sieve with 0.5 mm. mesh is practically about the finest that will serve for the 
dry separation with advantage, and since that same diameter is almost exactly that of 
quartz grains passing off at the maximum current-velocity conveniently available, viz., 
64 mm. per second, the writer has adopted the 0.5 mm. limit as practically the best for 
the fine earth to be used for both mechanical and chemical analysis. It is to such fine 
earth that the data hereinafter given are meant to apply. 

DISINTEGRATION BY BOILING. 

This is applicable to all soils, the time required varying greatly with different ones. 
Those containing much lime carbonate require the longest time to resolve those aggre- 
gates which will be also destroyed by tillage. Further than this the disintegration 
should not go ; nor should it fall seriously short of that normal measure. Thirty hours 
have in one case barely sufficed for a black calcareous prairie soil, while three have been 
found sufficient to reduce other soils to clean, single grains, as observed under the lens. 
While an absolute rule can not therefore be given, it may be said that with most soils 
from eight to fifteen hours will be the right measure, which with some may extend to 
twenty and twenty-four. The fact ascertained by Osborne, that the diff usibility of some 
clays, at least, is diminished by long boiling, renders it desirable to restrict its duration 
to a minimum. 



/>•* 



The boiling is best done in a thin, long-necked copper or glass flask of about 1 liter 
capacity, tilled four fifths full of distilled water and laid on a stand, on wire netting, at 
an angle of 40° to 45°. It is provided with a cork and condensing tube of sufficient 
length (5 to 6 feet) to condense all or most of the steam formed when ebullition is kept 
up by means of a gas flame. For a few hours the boiling generally proceeds quietly; 
but as the disintegration progresses violent bumping sets in, which sometimes endan- 
gers the flask, but is of assistance for the attainment of the object in view. In extreme 
cases some of the heavier sediment (generally clean sand) may be removed from the 
flask, but this is undesirable. 

It is frequently the case that when the boiled contents are left to settle, the liquid 
appears perfectly clear within an hour, although so soon as they are largely diluted the 
clay becomes diffused as usual, and will not settle in weeks. Probably this is owing to 
the extraction from the soil of soluble salts, which exert the same influence as does lime 
or common salt, even in very dilute solutions. 

REMOVAL OF THE COLLOIDAL CLAY AND FINEST SEDIMENTS. 

The boiled fluid with sediment is transferred to a beaker and diluted so as to form 
from 1 to \% liters in bulk, and being stirred up, is allowed to settle for such a length of 
time as (taking into account the height of the column) will allow all sediments of 0.25 
mm. hydraulic value to subside, the process being repeated with smaller quantities 
of water (distilled) until no sensible turbidity remains after allowing due time for 
subsidence. 

It must be remembered that this time is considerably longer than that for pure 
water, so long as any considerable amount of clay remains in the liquid, rendering it 
more viscous. And as the precise amount of allowance to be made can not in general be 
foreseen, some sediment of and exceeding 0.25 mm. hydraulic value, will almost inevita- 
bly be decanted with the successive clay waters, until the buoyant effect of the clay 
becomes insensible. The united clay waters (of which there will be from 4 to 8 liters) 
must therefore be again stirred up, and the proper time allowed for the sediments of 
0.25 mm. and over to subside. The dilution being very great, a pretty accurate separa- 
tion is thus accomplished ; the sediments being then ready for the elutriator. 

SEPARATION OF CLAY AND THE FINEST SILT IN THE "CLAY WATER." 

The now well-known property of colloidal clay, of remaining suspended in pure 
water for weeks and even months, offers an obvious method of separation from at least 
the greater portion of silts finer than 0.25 mm. hydraulic value «0.25). To push this 
separation to the extreme of attempting to remove all but the kaolinite particles proper, 
even were it feasible, would carry the time and labor required for the determination 
beyond the limits required for practical purposes, and would render the performance of 
mechanical analyses rare events in most laboratories. In special cases, of course, it 
may be desirable to go to these lengths, and also to divide the sediments lying between 
the clay proper and the finest sediment that can conveniently be obtained by the 
hydraulic method (0.25 mm. hydraulic value) into two or more groups, when it is very 
abundant. 

The clay water for subsidence is placed in a wide cylindrical or conical vessel (in 
which it may conveniently occupy 200 mm. in height); it is there allowed to settle for 
twenty-four hours. This interval of time was at first arbitrarily chosen, but it was 
subsequently found to be about the average time required by the finest siliceous silt 
usually present in soils, to sink through 200 mm. of pure water. So long as any sensible 
amount of clay is present, the time of course is longer, say from forty to sixty hours, or 
even more, if the clay be abundant and the liquid not very dilute. The sharp line of 
separation between the dark silt cloud below and the translucent clay water above is 
readily observed, and the time of subsidence regulated accordingly. At times, several 
such lines of division may be seen simultaneously in the column, indicating silts of 
successive sizes, with a break between. No such appearance is presented when, after 
weeks of quiet, the clay itself gradually settles. The liquid, which may be almost clear 
at the surface, then shades off downward very gradually, until near the bottom of the 
vessel it becomes entirely opaque. 



— 8 — 

After decantation of the clay water, the remaining liquid is poured off temporarily, 
leaving the sediment as dry as possible. It is then rubbed or kneaded in the decanting 
vessel itself, with a long-handled, soft-rubber pestle (conveniently cut out of a rubber 
cork). At this point the addition of a few drops of ammonia water, according to 
Schlosing's prescription, renders good service ; but' it is undesirable to use any large 
amount of ammonia, as it impedes the subsequent precipitation of the colloidal clay. 

Distilled water is again poured on (agitating as much as possible to break up the 
molecular aggregates) to the proper height, and another twenty-four hours' subsidence 
allowed. This operation is repeated six to nine times, until either the water remains 
almost clear after the last subsidence, or the decanted turbid water fails to be precipi- 
tated by salt water, showing the suspended matter to be pulverulent silt only. 

Doubtless the fine silt obtained in the twenty-four hours' subsidence, the diameter of 
whose quartz particles varies from 0.001 to 0.02 mm., is not entirely free from adherent 
colloidal clay, as is indicated by its deeper tint, compared with that of the coarser 
sediments; nor is the "clay" thus obtained free from the finest particles of quartz and 
especially of ferric oxid ; but it is doubtful whether for practical purposes a closer 
separation, such as was proposed by Williams,* is called for. 

The extent to which these contaminations exist, and the distribution of the impor- 
tant soil ingredients among the several sediments, have been discussed in other papers. 

Determination of the Colloidal Clay. — The colloidal portion of the kaolinite constitu- 
ent is of such preeminent importance that to throw upon it the indefinite " loss by 
analysis" and estimate it " by difference," is hardly excusable. Two ways of determin- 
ing it directly in the turbid waters from the twenty-four hours' subsidences are open to 
us. One is to evaporate the whole or an aliquot portion ; if the latter is not too small, 
and the soil is measurably free from the carbonates of lime and magnesia and other 
soluble salts, this method may yield fairly satisfactory results. But 100 cc. out of per- 
haps 20 liters of water, as has been practiced by some, is at best a rather minute base 
line to go upon in so important a determination ; moreover, it is so desirable to have 
the "clay " tangibly before one for examination, that we consider it altogether preferable 
either to evaporate the entire amount of clay water, which can readily be done pari 
passu witn the sedimentation, and then if necessary to extract the soluble portions with 
water or very dilute acid. I consider it best, however, to precipitate the clay by means 
of a saline solution and thus weigh the whole. The use of lime water, which naturally 
suggests itself, is so complicated by chemical reactions and other elements of uncer- 
tainty, that I have found it preferable to employ simply pure rock-salt brine for the 
precipitation. Fifty cc. of a saturated brine, i. e., 1.5 per cent of salt, is ordinarily 
sufficient to precipitate 1 liter of clay water; the precipitation is much favored by 
warming. Half the quantity, or even less, will do the same, but more time is required, 
and the precipitate is more voluminous. 

In practice it will be found desirable to thus precipitate each lot of clay water as 
soon as drawn off. The clay precipitate (which greatly resembles the usual iron- 
alumina precipitate of chemical analysis) will then at the end of twenty-four hours have 
shrunk into so small a bulk that on drawing off the supernatant liquid, the succeeding 
clay water from twenty-four hours' subsidence may be mixed with it, causing it to 
rediffuse; the same being done at each succeeding drawing off. This mode of operation 
greatly facilitates and shortens the gathering of the colloidal clay, which is precipitated 
much less easily and sharply from very dilute waters than from those heavily charged. 
As the clay precipitate can not ordinarily be washed with pure water, in which it 
quickly diffuses, it must be collected on a weighed filter, washed with a weak brine* 
dried at 100° and weighed. It is then again placed in a funnel and washed with a weak 
solution of sal ammoniac, until the chlorid of sodium is removed. The filtrate is 
evaporated, the residue ignited and weighed ; its weight, plus that of the filter, deducted 
from the total weight, gives that of the clay itself. 

In some cases, especially of clays and subsoils deeply tinged with iron, the clay, after 
drying at 100°, will not readily diffuse in water, and can be washed with pure water 
until free from salt ; it can then, of course, be weighed directly. 



* Wollny's Forschungen a. d. Gebiete der Agrikulturphysik, vol. 18, p. 225. 



IV 

— 9 — 

Properties of Pure Clay.— The "clay " so obtained is quite a different substance from 
what usually comes under our observation as such, since its percentage seems rarely to 
reach 75 in the purest natural clays, 40 to 47 in the heaviest of clay soils, and 8 to 20 
in ordinary loams. Thin crusts of it are occasionally found in river bottoms, where 
clay water has, after an overflow, gradually evaporated in undisturbed pools. When 
freshly precipitated by salt it is gelatinous, resembling a mixed precipitate of ferric 
oxid and alumina. On drying, it contracts almost as extravagantly as the latter, crimp- 
ing up the filter, to which it tenaciously clings, and from which it can be separated 
only by moistening on the outside, when it may mostly, with care, be peeled off. After 
drying it constitutes a hard, often horny, mass, difficult to break, and at times some- 
what resonant. Since the ferric oxid with which the soil or clay may have been colored 
is mainly accumulated in this portion, it often possesses a correspondingly dark-brown 
or chocolate tint. When a large amount of iron is present water acts rather slowly on 
the dried mass, which gradually swells, like glue, the fragments retaining their shape. 
Not so when the substance is comparatively free from iron. It then swells up instantly 
on contact with water; even the horny scales adhering to the upper portion of the filter 
quickly lose their shape, bulge like a piece of lime in process of slaking, and tumble 
down into the middle of the filter. 

There is a marked difference, however, in the behavior to water of clays equally free 
from ferric oxid, some exhibiting the phenomena just described in a more energetic 
manner than others. On the whole, those freest from iron appear to imbibe the water 
and crumble most readily. As this property possesses highly important bearings, both 
on the agricultural and ceramic qualities of clays, we propose to investigate it more 
minutely hereafter. 

The pure clay, when dry, adheres to the tongue so tenaciously as to render its sepa- 
ration painful. When moistened and worked into the plastic condition, it is exceed- 
ingly tenacious and " sticky," adhering to everything it touches. 

Under a magnifying power of 350 diameters no definite particles can be discovered 
in the opalescent clay water remaining after several weeks' subsidence. The precipitate 
formed by saline solutions then appears as an indefinite cloud (mostly of a yellowish 
tinge), for which one vainly seeks a better focus. In stronger clay water, or with 
higher magnifying powers, one can discern a great number of indefinite punctiform 
bodies, very uniformly diffused throughout the liquid, showing active "Brownian 
motion," and apparently opaque; the precipitate then formed by brine also shows a 
faintly dotted structure of its clouds.* 

Chemical Nature of the Clay Precipitate. — While usually considered as consisting 
essentially of kaolinite substance in a state of extremely fine division, the colloidal clay 
doubtless contains in most, if not in all, cases, other colloidals or "hydrogels," whose 
absorptive functions (albeit not plasticity) are in a measure similar to those of clay. 
Since in many cases the silica set free by treatment of the precipitate with acid is 
materially below that of the alumina dissolved by the same treatment, it follows that 
free aluminic hydrate is then present. The colloidal ferric hydrate, likewise, is accu- 
mulated in the clay precipitate, and so are amorphous zeolitic compounds. While it is 
thus certain that the most careful mechanical separation of this clay can give only an 
approximation to the really plastic kaolinite substance, yet such approximation is 
infinitely closer than that attained by determination of total alumina by boiling 
sulfuric acid, still sometimes prescribed in text-books. As in such treatment all the 
chalky kaolinite particles are also decomposed, it does not lead to even the roughest 
approximate estimate of the soil's plasticity. t It is of late claimed by many that the 
latter is merely a function of extremely fine subdivision. But no one who has handled 
the extremely fine portions deposited from the turbid water from quartz mills in the 
form of " slickens " can fail to appreciate the fact that even though these quartz pow- 
ders may remain in suspension as long as the clay itself, they do not remotely approach 
in plasticity even to an ordinary clay. We may be unable to define the physical nature 
of plasticity, but it certainly does not belong as such to all fine powders, nor to other 
gelatinous bodies. 

* According to Williams and Whitney the finest particles of colloidal clay are 0.0001 mm. diame- 
ter. Wollny gives .001 mm. as the limit, stating that the particles have the form of short, straight 
spicules. Probably the subject is far from its final form as yet. 

t See, on these points, my article on " The determination of clay in soils." Agr. Science, 6, 156. 



— 11 — 

TREATMENT OF THE COARSER SEDIMENTS. 

The mixed sediments remaining after the separation of the clay, and of silts of less 
than 0.25 mm. hydraulic value by decantation, are ready for the elutriator, regarding 
which some general conditions have already been given; the main point to be guarded 
being the prevention of the formation of liocculent aggregates out of the granules of 
the finer sediments. 

The Elutriator. — The following is a description of the instrument (see plate) as devised 
by me for the purpose of breaking up these flocculent aggregates; also of the simpler 
form (Schone's elutriator as modified by me), which can serve for grain-sizes above 
8 mm. hydraulic value (the latter is conveniently selected as to have half the cross- 
section of the former, so that with the same position of the index lever the velocity will 
be just doubled). A cylindrical glass tube, of about 45 mm. inside diameter at its 
mouth and 290 to 300 mm. high, has attached to its base a rotary churn, consisting of 
a brass cup, shaped like an egg with point down, so as to slope rather steeply at base, 
and triply perforated, viz.. at the bottom for connection with the relay reservoir; at 
the sides, for the passage of a horizontal axis bearing four grated wings. This axis, 
of course, passes through stuffing boxes provided with good thick leather washers, 
saturated with mutton tallow. These washers, if the axis runs true, will bear many 
millions of evolutions without material leakage ; when a beginning is noted additional 
washers may be slipped on, without emptying the instrument, until the analysis is 
finished. From five to six hundred revolutions per minute is a proper velocity for the 
finest sediments, which may be imparted by clockwork, turbine, or electric power. The 
driving pulley should not be directly connected with the axis, both because this is liable 
to cause leakage, and because it is necessary to be able to handle the elutriator quickly 
and independently. This is accomplished by the use of "dogs" on the pulley and 
churn axis. For the grain-sizes of 1 to 8 mm. hydraulic value, lower velocities are 
sufficient. Too low a velocity causes an indefinite duration of the operation, and may 
be recognized by the increase of turbidity as the velocity is increased. 

As the whirling agitation caused by the rotation of the dasher would gradually com- 
municate itself to the whole column of water and cause irregularities, a wire screen of 
0.8 mm. aperture is cemented to the lower base of the cylinder. 

The relay vessel below the churn of the elutriator tube should be a thick, conical 
test glass with foot. Its object is to serve as a reservoir for the heavy sediments not 
concerned at the velocity used in the elutriator tube, and whose presence in the latter, 
or in its base (the churn), would only cause abrasion of the grains and changes of cur- 
rent velocity, such as occur in the apparatus of Schone, and compel the current measure- 
ment of the water delivered. It is connected above with the churn by a brass tube 
about 10 mm. in clear diameter, so as to facilitate the descent of the superfluous sedi- 
ments, which the operator, knowing the proportion of area between the connecting 
tube and elutriator, can carry to any desired extent, thus avoiding the disturbance of 
the gauged current velocities as well as all material abrasion. 

A glass delivery tube should extend quite half way down the sides of the relay vessel 
to insure a full stirring up of the coarse sediments when required. By means of a rub- 
ber tube, not less than 20 inches in length, this delivery tube connects with a siphon 
carrying the water from near the bottom of a Mariotte's bottle— a 10-gallon acid-carboy. 
A stopcock, provided with a long, stiff index lever moving on an empirically graduated 
arc, regulates the delivery of water through the siphon. Knowing the area of the cross- 
section of the elutriator tube, the number of cubic centimeters of water which should pass 
through it in one minute at 1 mm. velocity is easily calculated ; and from this the lever 
positions corresponding to other velocities are quickly determined and marked on the 
graduated arc. The receiving bottle for the sediments must be wide and tall, so as to allow 
the sediment to settle while the water flows from the top into the waste pipe. The 
receiving funnel tube must dip nearly to the bottom of the bottle. 

Thus arranged the instrument works very satisfactorily, and by its aid soils and clays 
may readily be separated into sediments of any hydraulic value desired. But in order 
to insure correct and concordant results it is necessary to observe some precautions, 
to wit : 



— 12 — 

(1) The tube of the instrument must be as nearly cylindrical as possible, and must be 
placed and maintained in a truly vertical position. A very slight variation from the 
vertical at once causes the formation of return currents, and hence of fiocculent aggre- 
gates on the lower side. 

(2) Sunshine, or the proximity of any other source of heat, must be carefully 
excluded. The currents formed when the instrument is exposed to sunshine will vitiate 
the results. 

(3) The Mariotte's bottle should be frequently cleansed, and the water used be as 
free from foreign matters as possible. For ordinary purposes it is scarcely necessary to 
use distilled water. The quantities used are so large as to render it difficult to main- 
tain an adequate supply, and the errors resulting from the use of any water fit for 
drinking purposes are too slight to be perceptible, so long as no considerable devel' 
opment of the animal and vegetable germs is allowed. Water containing the slimy 
fibrils of fungoid and moss prothalia, algse, vorticellse, etc., will not only cause errors by 
obstructing the stopcock at low velocities, but these organisms will cause a coalescence 
of sediments that defies any ordinary churning and completely vitiates the operation. 

(4) The amount of sediment discharged at any one time must not exceed that pro- 
ducing a moderate turbidity. Whenever the discharge becomes so copious as to render 
the moving column opaque, the sediments assume a mixed character, coarse grains 
being apparently upborne by the multitude of light ones, whose hydraulic value lies 
considerably below the velocity used ; while the churner also fails to resolve the molec- 
ular aggregates which must be perpetually re-forming where contact is so close and 
frequent. This difficulty is especially apt to occur when too large a quantity of mate- 
rial has been used for analysis, or when one sediment constitutes an unusually large 
portion of it. Within certain limits the smaller the quantity employed the more con- 
cordant are the results. Between 15 and 20 grams is the proper amount for an instrument 
of the dimensions given above. 

THE FINE SEDIMENTS (0 25 TO 4 MM. HYDRAULIC VALUE). 

It has been found that, practically, 0.25 mm. per second is about the lowest velocity 
available within reasonable limits of time, and that, by successively doubling the veloci- 
ties up to 64 mm., a desirable ascending series of sediments is obtained, provided 
always that a proper previous preparation has been given to the soil or clay. It would 
seem that, according to the prescription given above for the preliminary sedimentation, 
no sediment corresponding to 0.25 mm. velocity should remain with the coarser portion. 
That such is nevertheless always the case, often to a large percentage, emphasizes the 
difficulty, or rather impossibility, of entirely preventing or dissolving the coalescence of 
these fine grain-sizes by hand stirring, as in "beaker elutriation." It is only by such 
energetic motion as is above prescribed that this can be fully accomplished, and the 
delivery of silts of 0.25 and 0.50 mm. hydraulic value really exhausted. 

The operation is best begun by turning on a low velocity, 0.25 to 0.50 mm., and then 
quickly rinsing the sediments from a small beaker into the elutriator before the column 
reaches the top. The latter is then quickly closed and a few seconds' subsidence allowed 
with diminishing velocity, so that the turbid column shall not be within less than 
30 mm. of the top when the velocity desired is turned on. Otherwise mixed sediments 
may pass at the beginning. At first the sediment passes off rapidly, and the column 
remains obviously and evenly turbid from the point where the agitation caused by the 
churner ceases to the top. But this obvious turbidity generally exhausts itself in the 
course of a few hours, and it then requires some attention to determine the progress 
of the operation. We have never known the 0.25 mm. sediment to become properly 
exhausted in less than fifteen hours, and in one case it has required ninety. The more 
rigorously the process of preliminary disintegration above described has been carried 
out, the shorter the time required for running off the fine sediments, which otherwise 
tax the operator's patience severely. As a matter of fact they never do give out entirely, 
doubtless for the reason that the stirrer continues to disintegrate compound particles 
which had resisted the boiling process. Besides, downward currents on the sides of the 
vessel will form, despite all precautions, so that the interior surface of the cylinder some- 
times becomes coated with Dendent flakes of coalesced sediment. These must then 



t!>\ 



— 13 — 

from time to time be removed by means of a feather, so as to bring them again under 
the influence of the stirrer; but it is, of course, almost mathematically impossible that, 
under these circumstances, any of the sediments subject to coalescence should ever 
become completely exhausted. Practically, the degree of accuracy attainable at best 
renders it unnecessary to continue the operation beyond the point when only a milli- 
gram or two of sediment comes over with each liter of water. It is admissible and even 
desirable to run off rapidly the upper third of the column at intervals of twenty minutes, 
whereby not only time is gained, but also the sediment in the relay reservoir is stirred 
and brought under the influence of the churner for more complete disintegration. 

It is noticeable that recent sediments, river-alluvium, etc., are much more easily 
worked than more ancient ones; as might be expected. Up to 4 mm. hydraulic value 
the use of the rotary stirrer is indispensable on account of the tendency to the forma- 
tion of compound particles. Beyond, this tendency measurably disappears, so that for 
the coarse sediments of 8 to 64 mm. hydraulic stirring may be employed and an elutri- 
ating tube of smaller diameter may advantageously be substituted in order to diminish 
the otherwise somewhat extravagant expenditure of water. The entire amount required 
for one analysis is from 25 to 30 gallons, provided a thorough previous disintegration 
has been secured. River water answers in ordinary cases; hard, spring or well waters 
are undesirable. Distilled water is of course best, and by a simple arrangement of an 
air-tight reservoir connected with a pressure chamber, can be returned to the Mariotte's 
bottle and used over and over. The average times required for the several sediments 
are as follows : 

Sediment: Hours. 

0.25 mm 30 to 40 

0.5 mm 15 to 25 

1.0 mm 5 to 10 

2 to 64 mm. 6 to 10 

Total 56 to 85 

With proper arrangements much of this can be done automatically at night, com- 
pleting an analysis (except the clay and finest silt determinations) in the course of 
three or four days. Of course only a very small portion of this time is given by the 
operator to the care of the instrument. He can carry on other work, just as when an 
evaporation is going on, with only an occasional glance to see that the water supply 
holds out and that there is no incipient leakage at the axis. 

As the soils are most conveniently weighed "dried at 100°,"* the sediments should be 
weighed in the same condition. Great care is necessary to obtain the correct weight 
of the extremely hygroscopic clay. The same is true, more or less, of the <0.25 sedi- 
ment, which, moreover, is so diffusible in water that it can not readily be collected on a 
filter. It is best, after letting it subside into as small a compass as possible, to evaporate 
the last 25-50 cc. in the platinum dish in which it is to be weighed. From the other 
sediments the water may be decanted so closely as to render their determination easy. 

The loss in the analysis of clays and subsoils containing but little organic or other 
soluble matter is usually from 1.5 to 2 per cent, resulting partially, no doubt, from the 
loss of the fine silt which comes off, more or less, throughout the process, and is 
decanted with the voluminous liquid. When the turbidity is marked, it indicates 
imperfect preliminary disintegration; it may be removed, and the silt collected, by 
adding a weighed quantity of alum, about 25 milligrams per liter, precipitating with 
ammonia, and deducting from the weight of the flocculent precipitate the calculated 
amount of alumina. 

The analysis of soils very rich in vegetable matter involves some modifications in 
the preliminary treatment and final weighings, which are discussed below. Ignition 
of the soil previous to elutriation, as proposed by some, is obviously inadmissible, as it 
would render impossible the separation of the clay from the finer sediments. 

As heretofore stated (Am. Jour. Sci., Dec, 1872 ; Proc. Am. Assoc. Adv. Sci., 1872, p. 71), 
the writer considers that, ordinarily, the investigation of the subsoils is better calculated 

* A somewhat clayey soil will continue to lose weight at 100° for five or six days. But after the 
first six hours the loss becomes insignificant for the purpose in question. 



— 14 — 

to furnish reliable indications of the agricultural peculiarities of extended regions than 
that of the surface soils, which are much more liable to local "freaks and accidents," 
and usually differ from the corresponding subsoils in about the same general points. 
For practical purposes, therefore, the difficulties incident to the physical analysis of 
soils rich in humus may in most cases be avoided. ' 

Character of the Sediments. — As regards the size of the particles constituting the suc- 
cessive sediments, the most convenient, because almost universally present, material 
for reference is quartz sand. Below is a table of measurements in which the values 
given refer to the largest and most nearly round quartz grains to be found in each sedi- 
ment. As a matter of course, all sizes between that given and the one next below are 
to be found in each sediment. A few grains of the finer sediments are also invariably 
present, owing both to the progressive disintegration of agglomerated particles by 
the stirrer, and to the inevitable formation of the flocculent aggregates of the finer 
sediments. 

While the measurement of the quartz grains (which are rarely wanting in a soil 
or clay) affords sufficient landmarks to the scientific observer, it seems desirable to 
attach to them, besides, generally intelligible designations, which shall approximately, 
at least, indicate the nature of the sediment. It is not easy to indicate in popular lan- 
guage distinctions not popularly made, but the grades of grain indicated in the common 
words, grits, sand, and silt, may, if numerically defined, serve at least to establish uni- 
formity of expression among scientific observers and reporters. Thus it might serve a 
useful purpose to apply the designation of "grits" to all grains above 1 mm. diameter 
up to "gravel." Below 1 mm. down to 0.1 mm. might be "sand," all below that "silt," 
viz. : impalpable powders. Then would follow clay, of which the distinctive character 
is not only impalpable fineness of grain, but also plasticity. To the analyst, however, 
the designations by hydraulic values will in the nature of the case always remain the 
most convenient, within the limits of the use of either sedimentation or hydraulic 
elutriation. 



Table of Diameters and Hydraulic Values of Sediments. 



Designation of Materials. 



Diameter 

of Quartz 

Grains. 



Velocity 
per Second, 

or 

Hydraulic 

Value. 



mm. 

Grits : 1-3 

Grits .5-1 

Coarse sand ._ ; .50 

Medium sand.. .30 

Fine sand .16 

Fine sand .12 

Coarse silt ... .072 

Coarse silt .047 

Medium silt .036 

Medium silt .025 

Fine silt .... .016 

Fine silt .010 

Clay (?) 



mm. 

(?) 

(?) 

64 

32 

16 

8 

4 

2 

1 

0.5 
0.25 
<0.25 
<0.0023 



It is noticeable that the absolute diameter of the elutriator tube exerts a sensible 
influence on the character of the sediments, in consequence of comparatively greater 
friction against the sides in a tube of small diameter. Strictly speaking, none of the 
sediments actually correspond to the velocity calculated from the cross-section of the 
tube and the water delivered in a given time, but to higher ones, whose maximum is in 
the axis of the tube, and which gradually decrease towards the sides, according to a 



— 15 — 

law which may be demonstrated to the eye by slightly diminishing the velocity while 
a sediment is being copiously discharged, so that the turbid column remains stationary 
while clear water is running off. The surface then assumes a paraboloid form, which is 
sensibly more convex in a tube of smaller diameter than in a wide one; the results 
obtained in the latter being, of course, nearest the truth. 

The sediments are conveniently preserved in homeopathic vials of uniform diameter; 
these, when arranged in a row, show a surface curve from which the prominent features 
of the physical composition of a soil may be seen at a glance. 

DETERMINATION OF THE WATER CAPACITY. 

This determination as usually made is very indefinite in its results, varying, 
especially in pervious soils, according to the height of the soil column in which the 
water is absorbed. This is due to the obvious fact that at the base of a soil column there 
is a maximum of this factor, decreasing regularly towards the top of the column, where 
it becomes a minimum. Now since the total ascent of water in columns of different 
soils varies from less than 375 mm. to over 2,800 mm., it is clear that any given 
uniform height of column arbitrarily agreed upon, as proposed by Ad. Mayer and 
others (e. g., 60 or 200 mm.), will give results standing in no direct rational relation to 
the maxima and minima of absorption by different soils. (See Wollny's Fortschr. Agr. 
Physik, 15, 1.) 

It is evident that in this case, as in others, either a maximum or a minimum deter- 
mination, or both, should be agreed upon • and such determination should be made in 
soil columns of as little height as possible ; that is, approaching as nearly as may be to 
the theoretical postulate of a mere differential. I suggest as the lowest practicable 
measure for this purpose a column of 10 mm., placed in a circular brass box with per- 
forated bottom, resembling the lead sieve of Plattner's blowpipe chest, and containing 
exactly 25 or 50 cc. In this both the maximum and minimum absorption is determined 
for each soil, proceeding as follows : 

Fill the box full of air-dried soil, of which the moisture content is determined in a 
separate portion at 100°. Settle the soil by a gentle tapping of the box on a table and 
then " strike " it level as in measuring grain ; weigh. 

Place the weighed box, plus soil, on a triangle submerged just beneath the surface of 
the water in a somewhat wide vessel; allow it to stand until fully saturated (not less 
than an hour, in order to insure the complete wetting of compacted soil particles); then 
wipe the sieve surface rapidly with filter paper or an absorbent towel and weigh again 
without unnecessary delay. Calculate from the weighings the maximum of water capac- 
ity with respect to both weight and volume. Now place the same vessel with wet soil 
on a flat desiccator plate, and immediately cover it with an additional (unweighed) 
quantity of the same soil, in order to absorb all the water down to the minimum of liquid 
absorption. 

The desiccator plate, as well as the bell covering it, must be lined with wetted blotting- 
paper in order to maintain a fully saturated atmosphere; and during the latter portion 
of the process of minimization at least, the soil used must itself have been previously 
exposed to such an atmosphere long enough to fully saturate it with hygroscopic moisture 
(see below), since otherwise such moisture would be taken up from the wetted soil and 
would thus vitiate the determination of liquid water capacity. A counterpoised glass 
plate on the scale pan of the balance serves to cover the soil cylinder while weighing. 

After the soil above has become imbued with moisture it is cut off level with the 
edges of the box by means of a tense, fine silk thread. The upper portion being thrown 
off, the operation is repeated until the fresh quantities of soil fail to become moistened 
above the edge of the box and cease to adhere, and weight ceases to decrease materially. 
Lastly, blow off carefully any loose particles remaining on the surface, and weigh. Dry 
the earth in the box thoroughly at 100° ; weigh. The water thus driven off will repre- 
sent the minimum of liquid absorption plus the hygroscopic moisture originally con- 
tained in the soil, as previously determined by drying at 100°. The soil mass will, as a 
rule, be found somewhat greater than that calculated from the first determination, 
because the unavoidable jarring and the weight of the soil poured on the wet mass 
compacts the latter to some extent. 



— 16 — 

By simple calculation the results may be referred to either of the two quantities for 
water capacity both by weight and volume, allowing for the increase of weight. 

It is clear that the soil so depleted of the water held in liquid absorption down to the 
minimum is in precisely the same condition as a similar layer of soil forming the top of 
a soil column in which the water has been allowed, to rise to its maximum height by 
capillary ascent. It seems at least probable that hereafter we may be able to deduce 
from these determinations the absolute height to which water will rise in a soil column 
in the course of time; but thus far we have no formula for such a deduction. Wollny 
has shown (Forsch. Agr. Physik, 8, 197) that, as a general rule, the differences between 
the maxima and minima become less as the soil becomes more fine-grained; and this 
difference becomes an important physical datum in judging of the capillary efficiency 
of the soil. We can then by the above method attain in two or three days results which 
by direct trial would require at least as many months ; and the differences between the 
maxima and minima thus ascertained far exceed those thus far on record. 

METHOD OF CHEMICAL SOIL ANALYSIS. 

The methods of chemical soil analysis hereinafter given are essentially those which 
in their main features have been pursued by Drs. David Dale Owen and Robert Peter 
in the work of the geological surveys of Kentucky and Arkansas, and which have since 
been further developed by the writer in the soil work of the surveys of Mississippi and 
Louisiana; in that done in connection with the Tenth Census "Report on cotton 
culture" throughout the cotton States; in that of the Transcontinental Survey, in the 
States of Oregon, Washington, and Montana ; and in the soil work of the California 
experiment station. Altogether these methods of soil investigation have been applied 
to over a thousand virgin soils, the analyses of which are strictly comparable among 
themselves; therefore by far the largest uniform set in existence thus far. Considering 
the labor involved in the work so performed, it becomes a serious question whether 
there are any valid reasons why the methods employed in it should be changed in any 
essential points, since to do so would throw out of any possible comparison with future 
work the bulk of what has heretofore been done in soil investigation on the North 
American continent. 

It is for this reason that the methods are given in considerable detail, not with a view 
of contesting the validity or equal reliability of other processes, but in order that every 
possible objection may now be raised, before more work shall be placed in jeopardy of 
being thrown out of comparison by a change of methods. Their convenience and 
substantial accuracy within the limits of personal error having been tested by consider- 
able experience, and the mode of interpreting the analytical results so as to accord with 
the farmers' experience in cultivation having been developed with respect to analyses 
made in accordance therewith, there should be strong reasons for changing the methods 
before such action is finally taken. In most respects the course of analysis here 
presented does not differ materially from that laid down by Kedzie in his report on the 
same subject; the chief differences arise in the preliminary operations, but are there 
quite vital, so as to necessitate an agreement if the comparability of analyses is to be 
maintained. 

It should be understood that these methods contemplate an approximation to the total 
maximum of solvent effect plants can exert upon the soil; carbonated water being by far too 
feeble as a solvent to represent even ordinary plant action, while hydrofluoric acid and 
fusion with alkalies go far beyond vegetative possibilities. It is certainly desirable that 
the limit of extraction should be a natural one rather than an arbitrary convention, 
which many chemists will be inclined to disregard when it manifestly does not fit the 
case before them. Experiments have shown that soils extracted with strong chlorhydric 
acid are permanently * sterile ; and if it can be shown that the results obtained from 
such soil extraction can be readily correlated with the permanent agricultural value of 
soils, the most important postulate of soil analysis is covered. 

PRELIMINARY PREPARATION OF THE SAMPLE. 

The soil is thoroughly broken up, dry, with a rubber pestle ; or in the case of clayey 
soils is digested with distilled water until fully disintegrated. A weighed quantity 



i. e , within the time interesting to the living generation. 



— 17 — 

(200 to 500 grams) is then sifted or washed, as the case may be, through a sieve of 0.5 mm. 
clear aperture. If washed, the muddy water must be evaporated to dryness with the 
soil slush and the whole thoroughly mixed. The sample so obtained constitutes the 
"fine earth" to be used in chemical analysis. The coarse portions are to be further 
segregated by sieves, weighed, and their mineralogical constituents identified with the 
microscope, reagents, or Thoulet's solution, as the case may require. 

That the introduction of the grain-sizes coarser than 0.5 mm. can as a rule serve no 
useful purpose in the chemical analysis of soils in the humid region is strikingly shown 
in the investigation made by R. H. Loughridge, in 1873 (Am. J. Sci., Jan. 1874, p. 17). 
He found that in the case of a very generalized soil of the Mississippi uplands, solution 
by strong acid practically ceased beyond the sediment of 0.5 mm. hydraulic value, cor- 
responding to a diameter of about 0.025 mm. Although the general applicability of 
this particular limit may be fairly questioned, the wide margins between the fractions 
0.5 and 0.025 renders it pretty certain that within the limit of the former we shall find 
all that is of any value to plants for their supply of mineral plant food. But for the 
sake of comparability with analyses made under a different rule, the grain-sizes of 
0.5 to 1 and 1 to 2 mm, diameter should always be quantitatively determined. 

It is true that in the arid region the larger grains contribute to plant nutrition, being 
themselves covered with a partially decomposed soil material. The investigations of 
Tolrnan* have, however, shown that such material is of the same general composition 
as the fine earth. If it be desired to make allowance for this fact by including the 
larger grain-sizes, it would be necessary to employ a correspondingly larger amount of 
soil for general anatysis. 

DETERMINATION OF THE HYGROSCOPIC COEFFICIENT. 

The fine earth is exposed to an atmosphere saturated with moisture for about twelve 
hours at the ordinary temperature ( p 0° P.) of the cellar in which the box should be 
kept. For this it is sifted in a layer of about 1 mm. thickness upon glazed paper, on a 
wooden table in a small water-tight covered box (12 by 9 by 8 inches) in which there is 
about an inch of water; the interior sides and cover of the box should be lined with 
blotting paper, kept saturated with water, to insure the saturation of the air. 

"Air-dried soil " yields results varying from day to day to the extent of as much as 
30 to 50 per cent, nor have we any corrective formula that would reduce such observa- 
tions to absolute measure. Knop's law, that the absorption varies directly as the tem- 
perature, while applicable to low percentages of saturation, is wide of the truth when 
saturation is approached. The observation of the writer has shown that between the 
temperatures of about 7 and 23° C. the coefficient of absorption in saturated air varies 
only by a small fraction ; hence the ordinary temperature of cellars will serve well in 
these determinations without material correction. 

After eight to twelve hours the earth is transferred as quickly as possible, in the 
cellar, to a weighed drying-bottle and weighed. The bottle is then placed in an air 
bath, the temperature of which is gradually raised to 110° C. and kept so for one hour. It 
is then weighed and the drying repeated until a practically constant weight is obtained. 
The loss in weight gives the hygroscopic moisture in saturated air. 

GENERAL ANALYSIS. 

The samples for general analysis and phosphoric acid determination are weighed out 
from the air-dried sample in which the hygroscopic moisture is afterward determined. 

In determining the amount of material to be employed for the general analysis 
regard must be had to the nature of the soil. This is necessary because of the impracti- 
cability of handling successfully such large precipitates of alumina as would result 
from the employment of even as much as 5 grams in the case of calcareous clay soils ; 
while in the case of very sandy soils even that quantity might require to be doubled in 
order to obtain weighable amounts of certain ingredients. For average loam soils in 
which the insoluble portion ranges from 60 to 80 per cent, 2.5 to 3 grams is about the 
right measure for general analysis, while for the phosphoric acid determination not less 
than 3 grams should be employed in any case. It has been alleged that larger quanti- 

* Report of the California Experiment Station for 1899-1900, page 33. 



— 18 — 

ties must be taken for analysis in order to secure average results. It is difficult to see 
why this should be true for the fine earth of soils and not for ores, in which the results 
affect directly the money value; while in the case of soils the interpretation of results 
allows much wider limits in the percentages. Correct sampling must be presupposed to 
make any analysis useful ; but with modern balances and methods it is difficult to see 
why 5 grams should be employed instead of half that amount, which in some cases is 
still too much for convenient manipulation of certain precipitates unless parting into 
aliquot portion is resorted to; which in the case of the iron-alumina precipitation 
would be very desirable, but can not usefully be carried out in practice. It is very much 
more difficult to secure a correct average sample when the coarser sizes of sand, such as 
1-2 mm. diameter, are retained in the fine earth, as these tend to settle down out of the 
finer portions. If, then, these larger sizes are insisted upon, 10 or more grams must be 
employed in order to secure a fair average sample. 

(1) The weighed quantity, usually of 2 to 2.5 grams, is brought into a small porcelain 
beaker covered with a watch glass, treated with 8 to 10 times its bulk of hydrochloric 
acid of 1.115 sp. gr. and 2 or 3 drops of nitric acid, and digested for five days over the 
laboratory steam bath. At the end of this time it is evaporated to dryness, first on the 
water bath and then on the sand bath. By this treatment all the silica set free is 
rendered insoluble. 

In an investigation made by Loughridge in 1873 (Am. J. Sci., Jan., 1874, p. 20) it 
was found that acid of the above strength exerted a higher solvent power than that 
materially stronger or weaker (1.100 or 1.160 sp. gr.). He also found that after the fifth 
day no further essential solvent action occurred. The above standard strength is easily 
prepared by steam distillation of acid either stronger or weaker. Later investiga- 
tions by Jaffa show that the effect of such acid approximates closely to that of oxalic 
acid, the strongest solvent available for plant-root action. We therefore determine in 
such treatment the maximum effect vegetation can exert on soils. 

The evaporation residue is now moistened with strong hydrochloric acid and 2 or 3 
drops of nitric acid, warmed, and, after allowing it to stand a few hours on the water 
bath, treated with distilled water. After clearing it is filtered from the insoluble residue, 
which is strongly ignited and weighed. If the filtrate should be turbid the insoluble 
residue which has gone through the filter can be recovered in the iron-and-alumina 
determination. 

The insoluble residue is next boiled for fifteen or twenty minutes in a concentrated 
solution of carbonate of soda, to which a few drops of caustic lye should then be added, 
to prevent reprecipitation of the dissolved silica. The solution must be filtered hot. 
The difference between the weight of the total residue and that of undissolved sand and 
mineral powder is recorded as soluble silica, being the aggregate of that set free by the 
acid treatment and that previously existing in the soil. The latter, however, rarely 
reaches 0.5 per cent. 

(2) The acid filtrate from the total insoluble residue is evaporated to a convenient 
bulk. In case the filtrate should indicate by its color the presence of any organic 
matter, it should be oxidized by aqua regia, otherwise there will be difficulty in separat- 
ing alumina. 

(3) The filtrate thus prepared is now brought to boiling and treated sparingly with 
ammonia, whereby iron and alumina are precipitated. It is kept boiling until the 
excess of ammonia is driven off, and then filtered hot (Mitscherlich method). (Filtrate 
A.) The previous addition of ammonic chlorid is usually unnecessary. If the boiling 
is continued too long, filtration becomes very difficult, and a part of the precipitate 
may redissolve in washing. Filtration may be begun so soon as the nose fails to note 
the presence of free ammonia; test paper is too delicate. Failure to boil long enough, 
or permitting the precipitated solution to cool, involves the contamination of the iron- 
alumina precipitate with lime and manganese. 

(4) The iron and alumina precipitate (with filter) of No. 3 is dissolved in a mixture 
of about 5 cc. hydrochloric acid and 20 cc. water. Then filter and make up to 150 cc. 
Take 50 cc. for the determination of iron and alumina together by precipitation with 
ammonia, after oxidizing the organic matter (filter) with aqua regia; also 50 cc. for 
iron alone ; keep 50 cc. in reserve. Determine the iron by means of a standard solution 
of permanganate of potash after reduction ; this latter is done by evaporating the 50 cc. 



— 19 - 

almost to dryness with strong sulfuric acid, adding water and transferring the solu- 
tion to a flask, and then reducing by means of pure metallic zinc in the usual way. 
The alumina is then determined by difference. This method of determining the two 
oxids in their intermixture is in several respects more satisfactory than the separation 
with alkaline lye, which, however, has served for most determinations made until 
within the last twenty years. It is much more liable to miscarry in unpracticed hands 
than the other. 

(5) The filtrate A. from iron and alumina is acidified slightly with HC1, and if too 
bulky is evaporated down to about 25 cc. (unless the soil is a very calcareous one) and 
the lime precipitated from it by neutralizing with ammonia and adding amnionic 
oxalate. The precipitation of the lime should be done in the hot solution, as the pre- 
cipitate settles much more easily. It is allowed to stand for twelve hours, then filtered 
off, washed with cold water, and dried (filtrate B ). By ignition the lime precipitate is par- 
tially converted into the oxid. It is then heated with excess of powdered ammonium 
carbonate, moistened with water, and exposed to a gentle heat (50°-80° C.) until all the 
ammonia is expelled. It is then dried below red heat and weighed as lime carbonate. 
When the amount of lime is at all considerable, the treatment with ammonic carbonate 
must be repeated until a constant weight is obtained. 

(6) The filtrate B from the calcic oxalate is put into a hard Bohemian or Jena flask, 
boiled down over the sand bath and the ammoniacal salts destroyed with aqua regia 
(Lawrence Smith's method). From the flask it is removed to a small beaker and evapo- 
rated to dryness with excess of HN0 3 . This process usually occupies four to five hours. 
The residue should be crystalline-granular; if white-opaque, ammonic nitrate remains 
and must be destroyed by HC1. 

The dry residue is now moistened with nitric acid and the floccules of silica usually 
present separated by filtration from the filtrate, which should not amount to more than 
10 or 15 cc. Sulfuric acid is then precipitated by treatment with a few drops of baric 
nitrate, both the solution and the reagent being heated to boiling. If the quantity of 
S0 3 is large, it may be filtered off after the lapse of four or five hours. If very small, 
let it stand twelve hours. The precipitate is washed out with boiling water, dried, 
ignited, and weighed (filtrate C). Care should be taken in adding the barium nitrate to 
use only the least possible excess, because in such a small concentrated acid solution the 
excess of barium nitrate may crystallize and will not readily dissolve in hot water. 
Care must also be taken not to leave in the beaker the large heavy crystals of baric 
sulfate, a few of which sometimes constitute the entire precipitate, rarely exceeding a 
few milligrams. Should the ignited precipitate show an alkaline reaction on moisten- 
ing with water, it must be treated with a drop of HC1, refiltered and weighed. The use 
of barium acetate involves unnecessary trouble in this determination. 

(7) Filtrate C from S0 3 precipitate is now evaporated to dryness in a platinum dish; 
the residue is treated with an excess of crystallized oxalic acid, moistened with 
water, and exposed to gentle heat. It is then ignited to change the oxalates to car- 
bonates. This treatment with oxalic acid must be made in a vessel which can be kept 
well covered with a thin watch glass, otherwise there is danger of loss through spattering. 
As little water as possible should be used, as otherwise loss from evolution of carbonic 
gas is difficult to avoid. Spatters on the cover should not be washed back into the basin 
until after the excess of oxalic acid has been volatilized. The ignited mass should have 
a slightly blackish tinge to prove the conversion of the nitrates into carbonates. White 
portions may be locally retreated with oxalic acid. The ignited mass is treated with a 
small amount of water, which dissolves the alkali carbonates and leaves the carbonates 
of magnesia, protosesquioxid of manganese, and the excess of barium carbonate behind. 
The alkalies are separated by filtration into a small platinum dish (filtrate D), and the 
residue is well but economically washed with water on a small filter. When the filtrate 
exceeds 10 cc, it may on evaporation show so much turbidity from dissolved earthy 
carbonates as to render refiltration on a minute filter necessary, since otherwise the 
soda percentage will be found too large, magnesia too small. If on dissolving the 
ignited mass the solution should appear greenish from the formation of alkaline man- 
ganates, add a few drops of alcohol to reduce the manganese to insoluble dioxid. The 
residue of barium, magnesium, and manganese compounds is treated on the filter with 
hydrochloric acid, and the platinum dish is washed with warm nitric acid (not hydro- 



— 20 — 

chloric, for the platinum dish may be attacked by chlorin from the manganese oxid), 
dissolving any small traces of precipitate that may have been left behind. 

(8) The solution containing the chlorids of magnesium and manganese is freed from 
the barium salts by hot precipitation with sulfuric acid, and the barium sulfate 
after settling a few hours is filtered off. The filtrate is neutralized with ammonia, any 
resulting small precipitate (of iron) is filtered off, and the manganese precipitated with 
bromine water. Let stand twelve hours (or over night) and filter (filtrate E); wash 
with cold water, dry, ignite, and weigh as manganese protosesquioxid Mn 3 4 . 

(9) From the filtrate E (from the manganese), the magnesia is precipitated by adding 
an equal bulk of ammonia water and then sodic phosphate. After standing at least 
twenty-four hours, the magnesia salt may be filtered off, washed out with ammoniacal 
water, dried, ignited, and weighed as magnesium pyrophosphate. 

(10) The filtrate E, which should not be more than 10 or 15 cc, containing the car- 
bonates of the alkalies, is evaporated to dryness and gently fused, so as to render 
insoluble any magnesium carbonate that may have gone through ; then redissolved 
and filtered into a small weighed platinum dish containing a few drops of dilute 
hydrochloric acid, to change the carbonates into chlorids ; evaporated to dryness) 
exposed to a gradually rising temperature (below red heat), by which the chlorids are 
thoroughly dried and freed from moisture, so as to prevent the decrepitation that would 
otherwise occur on ignition. Then, holding the platinum basin firmly by forceps 
grasping the clean edge, pass it carefully over a very low Bunsen flame, so as to cause, 
successively, every portion of the scaly or powdery residue to collapse, without fully 
fusing. There is thus no loss from volatilization, and no difficulty in obtaining an 
accurate, constant weight. The weighed chlorids are washed by means of a little water 
into a small beaker or porcelain dish, treated with a sufficient quantity of platinic 
chlorid, and evaporated to dryness over the water bath. The dried residue is treated 
with a mixture of 3 parts absolute alcohol and 1 part ether, leaving the potassio-platinic 
chlorid undissolved. This is put on a filter, and washed with ether-alcohol. When 
dried, the precipitate and filter are put into a small platinum crucible and exposed to a 
heat sufficiently intense to reduce the platinum chlorid to metallic platinum and to 
volatilize the greater part of the potassium chlorid. This is easily accomplished in a 
small crucible, which is roughened by being constantly used for the same purpose (and 
no other), the spongy metal causing a ready evolution of the gases. (See Fres. Ztschr. 
anal. Chem., 1893.) The reduced platinum, which should adhere firmly to the crucible 
after ignition over a blast lamp for twenty minutes, is now first washed in the crucible 
with hot acidulated water, then with pure water; then all moisture is driven off and it 
is weighed. From the weight of the platinum is calculated the potassic chlorid and 
the oxid corresponding; the difference between the weights of the total alkali chlorids 
and potassic chlorid gives the sodic chlorid, from which may be calculated the sodic 
oxid. When the heating of the platinum precipitate has not been sufficient in time or 
intensity, instead of being in a solid spongy mass of the color of the crucible itself, 
small black particles of metallic platinum will obstinately float on the surface of the 
water in the crucible, and it becomes difficult to wash by decantation without loss. 

PHOSPHORIC ACID DETERMINATION. 

(11) The weighed quantity (usually of 3 to 5 grams) is ignited in a platinum crucible, 
care being taken to avoid all loss by dusting. The loss of weight after full ignition gives 
the amount of chemically-combined water and volatile and combustible matter. 

(12) The ignited soil is now removed to a porcelain or glass beaker, treated with four 
to five times its bulk of strong nitric acid, digested for two days, evaporated to dryness 
first over the water bath and then over the sand bath, moistened with nitric acid, heated 
and treated with water. After standing a few hours on the water bath it is filtered off 
from the insoluble residue and the filtrate is evaporated to a very small bulk (10 cc), 
and treated with about twice its bulk of the usual ammonium molybdate solution, thus 
precipitating the phosphoric acid. After standing at least twelve hours, at first at a 
temperature of about 50° O, it is filtered off and washed with a solution of ammonium 
nitrate acidified with nitric acid. The washed precipitate is dissolved on the filter with 
dilute ammonia water. After washing the filter carefully the ammoniacal solution is 



— 21 — 

treated with magnesia mixture, by which the phosphoric acid is precipitated. After 
allowing it to stand twenty-four hours it is filtered off, washed in the usual way, dried, 
ignited, and weighed as magnesium pyrophosphate, from which the phosphoric acid is 
calculated. The per cent of phosphoric acid found is to be subtracted from that of the 
alumina. When a gelatinous residue remains on the filter after dissolving the molybdo- 
phosphate with ammonia it may consist either of silica not rendered fully insoluble in 
the first evaporation, or, more rarely, of alumina containing phosphate. It should be 
treated with strong nitric acid, and the filtrate with ammonic molybdate; any precipi- 
tate formed is of course added to the main quantity before precipitating with magnesia 
solution. 

HUMUS DETERMINATION IN SOILS. 

The estimation of " humus" by combustion, in any form, of the total organic matter 
in the soil gives results varying according to the season, and having no direct relation 
to the active humus of the soil. The same objection lies against extraction with strong 
caustic lye, which quickly humifies additional organic matter present. To obtain an 
estimate of the humus actually active in plant nutrition through nitrification, we must 
eliminate the unhumified organic matter. The best mode for accomplishing this, thus 
far known, is : 

GRANDEATj'S METHOD. 

About 10 grams of soil is weighed off into a prepared filter. The soil should be 
covered with a piece of paper (a filter) so as to prevent it from packing when solvents 
are poured on it. It is now treated with hydrochloric acid from 0.5 per cent to 1 per 
cent strong (25^ cc. of strong acid and 808 cc. of water) to dissolve out the lime and 
magnesia which prevent the humus from dissolving in the ammonia. Treat with the 
acid until there is no reaction for lime ; then wash out the acid with water to neutral 
reaction. Dissolve the humus with weak ammonia water, prepared by diluting common 
saturated ammonia water (178 cc. ammonia to 422 cc. water). Evaporate the humus 
solution to dryness in a weighed platinum dish at 100° C; weigh, then ignite; the loss 
of weight gives the weight of humus, the matiere noire of Grandeau. 

The examination of the ash of this humus, which contains notable amounts of phos- 
phoric acid, potash, lime, and magnesia, does not appear to offer sufficient information 
to justify its analysis in ordinary cases. 

DETERMINATION OP NITROGEN IN SOILS. 

The humus determination has been thought to indicate approximately the store of 
nitrogen in the soil, which must be gradually made available by nitrification. Ordina- 
rily (outside of the arid regions) the determination of ammonia and nitrates present in 
the soil is of little interest for general purposes, since these factors will vary with the 
season and from day to day. Kedzie (in the report above quoted) proposes to estimate 
the active soil nitrogen (ammonia plus nitrates and nitrites) by treatment of the whole 
soil with sodium amalgam and distillation with lime. The objection to this process is 
that the formation of ammonia by the reaction of the alkali and lime upon the humus 
amides would greatly exaggerate the active nitrogen and lead to a serious overestimate 
of the soil's immediate resources. 

The usual estimate of nitrogen in black soil-humus (Grandeau's matiere noire, de- 
termined as above) is from 5 to 6 per cent in the regions of summer rains. From late 
determinations it would seem that in the arid regions the usually small amount of humus 
(often less than 0.20 per cent) is materially compensated by a materially higher nitrogen 
percentage. It thus becomes necessary to determine the humus-nitrogen directly ; and 
this is easily done by substituting in the Grandeau process of humus-extraction, potash 
or soda lye for ammonia water, and determining the nitrogen by the Kjeldahl method 
in trie filtrate. While it is possible that the ammonia water would not vitiate the 
determination of nitrogen in the case of neutral or calcareous soils, it would certainly 
do so in the case of those having an acid reaction. 

The lye used should have the strength of 4 per cent in the case of potassic hydrate, 
3 per cent in that of sodic hydrate. The black humus filtrate or an aliquot part is 
placed in the Kjeldahl flask with concentrated sulfuric acid, and the nitrogen deter- 
mined in the usual way. 



— 22 — 

To this method the objection has been made that the "humus" extracted by fixed 
alkalies is not necessarily the same as that dissolved by ammonia. This objection may 
be obviated by extracting two separate portions (of 10 grams) of the soil with ammonia, 
as above, and using the one for the determination of total humus; while the other' 
after evaporation to small volume, is mixed with about 5 per cent of magnesic oxid, 
which in boiling eliminates all the ammonia that has been taken up from the ammoniacal 
solvent; after which the nitrogen in the residue is determined by the usual Kjeldahl 
method. The results obtained by these two methods, however, usually agree very 
closely. 

For the determination of nitrates in the soil it is, of course, usually necessary to use 
large amounts of material, say not less than 50 grams and, according to circumstances, 
five or more times that amount. In the evaporated solution the nitric acid is best 
determined by the reduction method, as ammonia. 

Usually the soil-filtrate is clear and contains no appreciable amount of organic 
matter that would interfere with the determination; yet in the case of alkaline soils 
(impregnated with carbonate of soda) a very dark-colored solution may be obtained. 
In that case the soil may advantageously be mixed with a few per cent of powdered 
gypsum before leaching; or the gypsum may be used in the filtrate to discolor it by the 
decomposition of sodic carbonate and the precipitation of calcic humate. The evapo- 
rated filtrate can then be used for the nitrate determination by either the Kjeldahl, 
Sprengel, or the colorimetric method, which will, of course, include such portions of 
the ammoniacal salts as may have been leached out. 

For the separate determination of these and of the occluded ammonia, when desired, 
it is probably best to mix the wetted soil intimately with about 10 per cent of magnesic 
oxid and distill off into titrated chlorhydric acid. For general purposes, however, this 
determination is usually of little interest. 

DETERMINATION OF "PROBABLY AVAILABLE PLANT FOOD." 
(Dyer's Method.) 

The methods of chemical soil analysis discussed in the preceding pages are intended 
to show the entire amount of mineral plant food likely to become available within the 
time interesting to the present and several succeeding generations, with a view to per- 
manent investments. In the case of virgin soils the adequate availability of these 
ingredients, and the consequent duration of productiveness, may usually be assumed to 
be in more or less direct proportion to the total amounts found. But in the case of soils 
long cultivated, it is extremely desirable to ascertain somewhat definitely the present 
needs of the soil, so as not to waste money on the purchase of unnecessary fertilizers. 
Numerous methods for this purpose have been suggested, from the extraction with 
large amounts of pure water to that with dilute chlorhydric acid, and various salts. Of 
all these methods none seems to accord so nearly with the results of practical culture as 
the extraction of the soil with a 2 per cent solution of citric acid, originally suggested 
by Maerker, and later more definitely developed by Dr. Dyer, whose name the method 
usually bears. Dyer's method, somewhat modified to conform to the conditions of the 
arid region, is as follows: 

Place in a flask, or bottle, 100 grams of air-dried soil and 600 cc. of distilled water con- 
taining 12 grams of pure citric acid. (Whenever the soil contains carbonates, a proper 
allowance must be made for them by a corresponding increase in the dose of citric acid.) 
The soil is left at room temperature in contact with the 2 per cent sc lution of citric acid 
for twenty-four hours, with frequent thorough shaking. At the end of the digestion the 
solution is filtered, and divided into three parts— one for the potash, and one for the 
phosphoric acid determination, the third as a reserve. Evaporate each to dryness, 
ignite, and determine the potash and phosphoric acid according to the methods given 
above for the general analysis of the soil. 

PRESENTATION AND INTERPRETATION OF RESULTS. 

It is strenuously suggested that in the presentation of the results of a soil analysis 
the order of the electrolytic series be observed, as in the schedule annexed, so as to 



— 23 — 

facilitate comparisons which are rendered unnecessarily troublesome by differences in 
arrangement. The insoluble residue is best placed at the head of the column.'as it 
indicates at a glance, approximately, the general character of the soil as sandy, loamy, 
or clayey. 



Coarse materials >>0.50 mm 
Fine earth 



100.00 
Chemical Analysis of Fine Earth. 

Insoluble matter _ 

Soluble silica - 

Potash (K 2 0) 

Soda(Na 2 0) 

Lime(CaO) 

Magnesia (MgO) 

Br. ox. of Manganese (Mn 3 4 ) 

Peroxid of Iron (Fe 2 (J 3 ) 

Alumina (A1 2 3 ) 

Phosphoric acid (P 2 5 ) ... .. 

Sulfuric acid (S0 3 ) _ 

Carbonic acid (C0 2 ) 

Water and organic matter 

Total 



Water-soluble matter, per cent. 
Chlorin, per cent 



Humus 

Ash 

" Nitrogen, per cent in Humus 
" " per cent in Soil 



Available Potash j citric acid 

Available Phosphoric Acid ] method 

Hygroscopic moisture, absorbed at ° C. 



Water capacity : By Volume. By Weight. 

Maximum _ _ 

Minimum 



For suggestions concerning the interpretation of analyses made according to the 
above methods, see "Report on the Experiment Station of the University of California," 
1890, pages 151 to 172.